Modern digital telecommunication systems are operating at ever-increasing data rates to accommodate society's growing demands for information exchange. However, increasing the data rates, while at the same time accommodating the fixed bandwidths allocated by the Federal Communications Commission (FCC), requires increasingly sophisticated signal processing techniques. Since low cost, small size and low power consumption are important in the hardware implementations of such communications systems, custom integrated-circuit solutions are important in achieving these goals.
Next-generation digital television systems such as proposed cable television (CATV) and high-definition television (HDTV) will rely on transceivers to deliver data at rates in excess of thirty megabits per second (30 Mb/s). Quadrature amplitude modulation (QAM) techniques, used in high-speed modems and digital ratio systems, represent a promising transmission format for CATV and HDTV systems. In quadrature amplitude modulation (QAM) systems, a pair of amplitude modulated signals having a quadrature (90°) phase relationship to each other are summed to transmit the television signals through the coaxial cable.
There are problems in the use of quadrature amplitude modulation for CATV and HDTV systems. One significant problem is that a considerable amount of noise and distortion is generated in the coaxial cables. Such distortion may result in CATV systems in part from impedance mismatches and reflections from unterminated stubs. In HDTV systems, the distortion may result in part from multi-path reflections. Such distortion is so significant that it impairs a good reception of the television signals.
Until now, analog systems have been proposed to recover the quadrature amplitude modulated data from the analog CATV and HDTV signals in the coaxial cables. Such systems have been disadvantageous because they have not been able to eliminate a significant amount of the noise and distortion in the coaxial cables. Even with their inefficiencies, they have required large amounts of power and considerable space.